This invention relates to the removal of undesired components from fluids and has particular relationship to such removal where the fluids flow at high velocities between about 1,000 and 2,500 feet per minute. While in the interest of being concrete in dealing with the details of this invention, this application concerns itself with the removal of undesired components from gas, it is also applicable, in its broader aspects, to the removal of undesired components from liquids. To the extent that this invention is so applicable to liquids, such application is within the scope of this invention. The components which are removed from a gas may be soluble such as soluble salts or acids, for example acetic, nitric or hydrochloric acid, or insoluble particulate such as sulfur, carbon powder, fly ash or graphite. The components may also be mist, either initially present or deliberately, injected into the gas to dissolve soluble particulates and to wet and wash down insoluble particulate. Essentially, this invention serves for gas scrubbing and/or mist elimination for high velocity gas flow.
Separation and removal of undesirable or contaminant particulates from a gas stream in accordance with the teachings of the prior art has been effected by two principal methods: gas filtering and centrifugal force separation. Gas filters are comprised of porous, foraminous or fibrous media, woven or nonwoven through which the gas is wholly conducted. Filters separate by means of inertial impaction and impingement of the particles on the fibers as the gas containing the particles passes through the filters. For removal of small particle sizes, the fibers must be fine. While gas filters are reasonably effective in particulate removal, the gas velocity through a filter must be maintained at relatively low levels to avoid excessive and uneconomic gas pressure drops. This is particularly true in the case of fine particulate filtration, where the particles are in the 1-20 micron size range and the fibers are very fine, typically in the 10 to 200 micron range. Gas pressure drop through a fibrous filter is approximately proportional to the square of velocity of the gas in the turbulent flow region. Industrial fine particulate filters, such as baghouses, consequently have very large filter surfaces, operating typically at 5 to 50 feet per minute gas face velocity, and are relatively expensive. Further, the optimum ranges of gas velocity through the filter media required to achieve efficient particle removal by the mechanisms of interception and impaction are invariably higher than those which can be practically and economically employed because of the gas-pressure drop across the filter which results at such higher velocities.
While fibrous or filamentary woven or felted gas filter media are applicable to particulate removal, including mist filtraion, they do not readily lend themselves to continuous washing in the case of solid particulate removal or to too high liquid loads in the case of mist removal alone. Because of the uniformity and small size of the fluid passages through such filter media, and the competition of liquid and gas flow for these fluid flow passages, the flow capacity of the filter for gas is restricted because of the presence of liquid, or that for liquid because of the presence of the gas. If a filter medium is used which possesses even a moderate degree of dynamic capillarity, under a specific combination of gas and liquid loadings, liquid is retained and gas flow choked off except at extremely high gas-pressure drops. Dynamic capillarity, or the equivalent dynamic liquid holdup in a medium, is the tendency to retain liquid in the pores of a medium, which exists under flow conditions of the gas and the dynamic loading rate by such flow of the medium with liquid.
"Dynamic loading rate" refers to the liquid holdup in a pervious medium such as a filter under dynamic conditions; i.e., during gas and liquid flow through the medium. Quantitatively dynamic loading rate is expressed as the fractional volumetric liquid retention of the medium under flow conditions. When the retention reaches a constant level, the dynamic loading is said to be stable. Dynamic loading rate is distinguished from static loading rate. "Static loading" refers to liquid retention under zero flow of liquid and gas. A capillary medium has high static liquid loading; a non-capillary material has low static and low dynamic loading; a non-free draining, non-capillary material has low static and high dynamic loading.
The capillarity effect is enhanced by the continuous injection of liquid by the flowing gas into the medium. In the case of static capillarity, the only force counteracting the capillary force is gravity. In the case of dynamic capillarity, the capillary force is counteracted in addition by the force exerted by the gas and augmented by the continuous loading of liquid. The effect of appreciable liquid loadings on such capillary filter media resulting from dynamic capillarity is to make the media behave as a virtually solid wall with respect to gas flow. For dynamic liquid loads, such as are generated by filter washing, even a small degree of capillarity yields significant gas flow passage closure with liquid. Additionally, if the gas stream contains solid insoluble particulates, capillary filter media are highly susceptible to solid plugging.
Rebours, U.S. Pat. No. 3,733,789, which is typical of the prior art of this type, uses a sprayed tubular filter cloth to form a continuous stable film of washing liquor on the cloth through which the gas is "microsieved." Rebours' data on gas-flow resistance as a function of linear gas velocity illustrates the typically high-resistance/low-flow characteristics of the capillary filter media, whether woven cloth or compacted or felted fibrous material. Rebours' teaching is specific for liquid "micromists" and as pointed out by Rebours, solid insoluble particles unfailingly clog the filter after a few hours of operation.
Fairs, U.S. Pat. No. 3,135,592, also discloses an irrigated filter medium but in countercurrent liquid-gas flow. Fairs' gas velocities are about 15 feet per minute and fall into Rebours' range of 4 to 20 feet per minute representing essentially laminar, as distinct from turbulent flow, gas flow. Rebours and Fairs are limited to such low gas velocities because the gas-flow passages are clogged with liquid. Sprayed screen devices such as those of Alliger, U.S. Pat. No. 3,763,634, and Mare, U.S. Pat. No. 3,785,127, suffer from the same gas-flow limitations resulting from the necessity of trying to force both liquid and gas through uniform-openings restricted with respect to liquid and gas flow and/or capillary flow space in a filter medium with uniform openings. Lucas, U.S. Pat. No. 3,370,401, discloses a teaching similar to that of Fairs, except that the fibrous filter medium is deliberately operated in the flooded condition.
Centrifugal-force separators, such as devices with parallel-vane serpentine or sinusoidal-paths, or chevron or zig-zag passages, are used primarily for mist elimination, in clean gases not containing solid particulate. Such parallel vane separators are commonly used for removing liquid carryover in steam boilers, water-cooling towers, and in gas-liquid contacting apparatus such as distillation or fractionation towers, evaporators, gas-scrubbing apparatus and the like. Another area of application is the removal of mists from the air intakes of power turbines such as marine-power or propulsion plants. In such separators the fluids carrying the suspended matter is bent or deflected by the vanes and the suspended matter is ejected by centrifugal force. The force exerted on a particle of mass M is M(v.sup.2 /r), where v is the velocity of the particle and r the radius of the path. It is desirable that v should be high or r small.
Centrifugal-force impaction separators operate at much higher velocities than do filter media, and are used for low-load liquid mist removal or for batch particulate removal. In zig-zag sinusoidal-passage parallel-planar types of separators, the removed liquid must drain under the influence of gravity without accumulating within the passages or being otherwise subject to reentrainment in the gas. Liquid occurring either as a mist, or as a deliberately-introduced spray wash that has been removed and collected as drops or as a liquid film on surfaces exposed to the flowing gas, is subject to being dragged along in the direction of gas flow by gas friction and momentum transfer. The resulting liquid carrythrough or reentrainment lowers the overall liquid removal efficiency. Various expedients have been suggested to overcome this deficiency. Hosch, U.S. Pat. No. 1,616,802, describes a serpentine-passage separator having liquid-collection baffles protruding from the peaks of the corrugations. Other modifications of the sinusoidal or zig-zag-path vane separators are typically shown by Clark, U.S. Pat. No. 2,802,543, Sokolowski, U.S. Pat. No. 3,751,886, Hurlburt, Sr. et al, U.S. Pat. No. 3,757,498, Hill, U.S. Pat. No. 3,813,855, and Regehr, U.S. Pat. No. 3,849,095. These show the use of solid, planar-wall vanes, having various catchments or arrangements to drain the liquid removed. However, such protrusion catchments, particularly those opening up toward the upstream side, are subject to gas impact and momentum transfer to the collected liquid in the exposed pocket. The liquid is unprotected with respect to gas friction and momentum transfer and is picked up and entrained in the gas. On the other hand, turning the catchments or protrusions to the down-stream side serves to introduce low-pressure accelerated gas-flow regions immediately downstream of the liquid catchment, which serves to suck or aspirate the liquid out of the protective pocket into the stream. These deficiencies of prior-art apparatus result in limited liquid loading or handling capability and a tendency to reentrain liquid at relatively low gas velocities.
In accordance with the teachings of the prior art, there are also provided gas-permeable structures defining serpentine passages or channels for the gas. Typical of this prior art are Schaff, U.S. Pat. No. 2,567,030, and Brixius, U.S. Pat. No. 2,760,597. Schaff and Brixius disclose particulate filter panels consisting of alternating corrugated layers of paper, fly screen and similar materials. Such filters are for "dry" use, inasmuch as the capillary nature of the internal walls would prevent gas permeation under liquid irrigation or a mist load. The corrugated layers of the Schaaf and Brixius filters are horizontally disposed with alternate layers reversely corrugated. If an attempt were made to use such devices for mist removal or other wet application, liquid drainage would be seriously impeded and such devices would be highly inefficient. The filter panels of this prior art are specifically disposable--and are intended for one-time batch use until plugged, at which time such filters are either discarded or removed from service and re-worked.
It is an object of this invention to overcome the above deficiencies and drawbacks of the prior art and to provide apparatus for removing undesired components from gas flowing at high velocity which apparatus shall readily drain the impinging liquid and/or particulate, shall be readily permeable to the gas, shall not become clogged by liquids or particulates and shall not require frequent replacement.